Signals which a radar antenna receives (reception signals) include echoes from waves, echoes from rain or snow, besides echoes from target objects or land. For this reason, in order to suppress such unused echoes, processing for subtracting only a given amount of signal level from the reception signals (gain adjustment) has been conventionally carried out. JP2012-017995A and JP2012-017996A disclose radar devices which carry out such a kind of processing.
JP2012-017995A and JP2012-017996A disclose the radar devices which calculate and set a suitable threshold according to a distance from a ship concerned based on the reception signal.
JP07-159523A discloses a radar device for carrying out Doppler processing to the reception signal. The Doppler processing is to exactly detect target objects (e.g., other ships) with relative speeds by utilizing that frequencies of the reception signals differ according to the differences in speed (relative speeds) of the target objects with respect to the ship concerned.
Specifically, the radar device disclosed in JP07-159523A is provided with two circuits comprised of a plurality of filters which separate one reception signal from the other. Therefore, one of the circuits has a different center frequency of the filter from the other. As a result of the signal processing, the device only outputs either one of the circuits' outputs having larger amplitude.
However, like JP2012-017995A and JP2012-017996A, when only performing the threshold processing and not performing the Doppler processing (for example, when amplitudes of a clutter echo and a ship echo are about the same), a detection of the ship will be difficult.
On the other hand, in the Doppler processing, processing for suppressing signals which are continuous in a distance direction is performed to suppress clutters. Therefore, echoes from land may also be suppressed (see FIGS. 7A and 7B). In addition, in the Doppler processing, since removal processing is performed after separating the signals based on the relative speeds, echoes from clutters and target objects at the same speed may also be suppressed.